Diagnostic Ratios and Isotopic Fingerprints of Vehicular Ammonia Across Emission Standards via Dynamometer Chassis Experiments

Diagnostic ratio and isotopic fingerprint hold exceptional promise for improving emission estimates and source attribution of fugitive and complex ammonia (NH3) emissions in urban atmospheres. However, systematic research linking these markers specifically to vehicular NH3 tailpipe emissions across evolving emission standards remains elusive. Here, 12 light-duty gasoline vehicles spanning Euro-3 to Euro-6 emission standards were tested under both cold- and hot-start conditions using a dynamometer chassis. Instantaneous emissions of NH3, and coemitted pollutants were comprehensively measured to explore potential diagnostic ratios, while concurrently collecting tailpipe NH3 for isotopic analysis (δ15N-NH3). We found that both NH3/CO2 diagnostic ratios and δ15N-NH3 values are highly dependent on the emission standard. NH3/CO2 diagnostic ratios varied significantly from 0.07 (Euro 6) to 0.54 ppbv/ppmv (Euro 5), while δ15N-NH3 values remained relatively stable for Euro-3 (−1.5 ± 9.2‰), Euro-4 (−1.0 ± 13.3‰), and Euro-5 vehicles (−5.8 ± 7.3‰), but showed significant 15N depletion (−13.7 ± 7.0‰) Euro 6 vehicles with ultralow emissions. These findings demonstrate that static assumptions for these markers are inadequate for accurately modeling emissions from modern, evolving vehicle fleets. We thus propose a dynamic framework that utilizes these emission-standard-specific fingerprints to enable precise quantification of city-specific and technology-based vehicular NH3 emissions and their contribution to the urban NH3 budget.